Technical Abstract:
Enterobacter cloacae 501R3 shows promise as a biological control agent for damping-off of cucumber caused by Pythium ultimum. Enterobacter cloacae strain C10 is a mini-Tn5 Km transposon mutant of strain 501R3 that was deficient in colonization of cucumber seedlings, and significantly reduced in colonization of sunflower and pea seedlings relative to strain 501R3 in natural soil. In addition, populations of strain C10 were 150-fold and 6700-fold lower than those of strain 501R3 in cucumber rhizosphere, respectively, in two separate 42-day colonization assays carried out in natural soil. No dramatic differences were detected between strains 501R3 and C10 in suppression of damping-off of cucumber caused by Pythium ultimum. Molecular characterization of strain C10 indicated mini-Tn5 Km was inserted in a region of the E. cloacae genome with a high degree of DNA and amino acid sequence similarity to degS, a putative periplasmic serine protease. In Escherichia coli, DegS is required for RpoE mediated gene expression in response to extracytoplasmic stress, through cleavage of the anti-SigmaE factor, RseA. This study suggests a role for degS, and by extension, the RpoE-mediated stress response in colonization of certain plant species.